Sintering kinetics and properties of highly pure lead zirconate titanate ceramics
Lead zirconate titanate (PZT) has been widely applied in actuators and sensors due to its excellent piezoelectric and ferroelectric properties. However, impurities, one of the major problems involved in the mass production of PZT ceramics, have not attracted enough attention. In this thesis, investigations on the effects of impurities from the raw materials on the sintering and properties of lead zirconate titanate with a composition of PbZr0.53Tit.47O3 were conducted. The impact of starting materials, lead nonstoichiometry, and dopants was examined. The sintering behavior was monitored in a thermo-optical dilatometer and a kinetic-field approach was employed to obtain the activation energy for sintering of PZT ceramics with different compositions. The bulk ceramics were characterized in terms of microstructure, dielectric, piezoelectric and ferroelectric properties. At first, a comparison in sintering between the PZT samples prepared from industrially used (IM) and highly pure raw materials (HM) was made. Reduced sintering temperatures and higher densification rate were observed on the IM sample owing to the secondary phase on the grain boundary. Different electric properties of these two samples were also evidenced. To evaluate their contributions to the observed difference of IM and HM samples, various impurities, which were identified in the IM raw materials, were added in the highly pure samples. It was shown that the sintering was changed through the formation of charged vacancies (impurity Na, Fe, Al, Y) or melt phase (Si) with low melting point. The most important impurity species was identified as Na, Y and Si and their effect on the ceramics properties was investigated as a function of dopant concentration. They showed grain growth inhibition effect on PZT ceramics. The strongest effect was achieved by doping with Na. The grain size was reduced from 13µm of undoped PZT to 2µm at a doping level of 1mol%. The dielectric constant was increased with Na doping, which was attributed to the decreasing grain size. In addition, because of the oxygen vacancies caused by the Na doping, ""hard"" piezoelectric behavior and ferroelectric properties were observed. Rare earth impurity, such as Y, with a valence and ion radius between A site and B site elements in PZT lattice, results in a combination of ""soft"" and ""hard"" characteristics. Melt phase formed from Si showed deteriorated effect on the properties of PZT ceramics. Deviations from the stoichiometric composition could result when the impurities were not considered in the weight fractions of the raw materials. The lead content in the system was affected by the crystalline phase of starting components as well. PbO concentration was changed during calcination depending on the formation kinetics of intermediate lead titanate. Sintering temperatures were dramatically reduced and densification rates were strongly enhanced by the introduction of lead oxide excess. The reason was believed to be associated with the liquid phase formed by PbO during sintering because of its low melting point. Rapid densification was observed at low level of PbO excess. However, a sluggish rearrangement process with low densification rate occurred in the PZT with 3.0% PbO excess at a temperature below the melting point of PbO. A small force (as small as 0.1MPa) on the sample could result in rapid densification and an additional densification maximum was evidenced. The tetragonal lattice distortion in the lead deficient samples was verified by Rietveld refinement, from which internal stress was introduced and attributed to the high dielectric constant. Moreover, the increasing amount of lead deficiency could result in the segregation of ZrO2. It shifts the Zr/Ti ratio to the Ti-rich side, which may be attributed to a higher dielectric constant as well. The lead excess is favorable in improving both the microstructure and electrical properties of PZT ceramics. However, deteriorated properties were found in the samples in which the lead oxide excess is beyond a certain level (1.5 mol% PbO). The kinetic field diagram was constructed using the shrinkage data from the optical dilatometry. Different dependence of activation energy on the fractional density was observed. It is attributed to the variation in the activation energies in densification and grain growth. By fitting the iso-strain lines, activation energy between 350-360 kJ/mol was obtained for densification of pure HM PZT sample and samples doped with 0.5% Na or Si. Smaller values were acquired for grain growth. Although liquid phase was present in Si-doped samples and the ones with PbO excess, a better match can be achieved using a solid state sintering model. The enhanced inhomogeneity and the rapid densification were suggested to explain the difficulty in fitting the sintering curves of 1.5wt%PbO added sample.
Bayreuth, Univ., Diss., 2009